Semiconductors often include integrated devices fabricated using a substrate or wafer. Examples of typical materials include silicon and gallium arsenide. Semiconductor devices integrate various circuit elements, such as resistors, capacitors, transistors, inductors, insulators and different types of memory.
MEMS devices integrate small mechanical systems with semiconductors to form various devices, such as sensors (e.g., temperature, pressure, gas, moisture and motion sensors), accelerometers, valves, motors, actuators and micromirrors.
Electroplating is one method used in fabrication of electrical contact points for MEMS devices and in MEMS packaging. Electroplating may include selective or blanket deposition of metals. Compared to other coating methods, electroplating can accommodate a variety of process temperatures and deposition rates. Electroplating can also yield varied deposit morphologies to accommodate specific applications.
FIG. 1 (prior art) is an illustration of a typical semiconductor electroplating apparatus 100, which includes a vessel 102 with a reservoir containing an electrolyte solution 104, an anode 106 and a cathode 108. The cathode 108 and the anode 106 form an electrical circuit with the electrolyte solution 104 and a power supply 112.
The cathode 108 typically includes the semiconductor wafer to be metallized. The cathode 108 is held to a support 110 by a clamp. For precious metal electroplating, such as gold plating, the anode 106 is formed from a metal (such as titanium) that is coated with platinum.
The electrolyte solution 104 is selected according to the metal to be electroplated. In at least one example, the electrolyte solution 104 includes: a solution of copper sulfate for copper plating; or a different solution of sodium or potassium gold cyanide for gold plating.
Electroplating can be performed using either: inert anodes, such as titanium with a thin coating of platinum (platinized titanium); or soluble anodes. If electroplating using inert anodes, all of the deposited metal comes from the electrolyte solution. If electroplating using soluble anodes, the deposited metal comes from electrodissolution into the electrolyte solution of the metal being deposited from solid anodes of the same metal. Ideally, the mass of metal dissolved from the soluble anode exactly balances the amount of metal deposited. In one method, the soluble anodes are in contact with an inert supporting anode to facilitate electrical connection and replenishment of the soluble anodes as they are consumed.
In a system with a supporting inert anode (such as platinized titanium) and soluble anodes, such as for indium plating, slow consumption of the platinum coating may occur to expose the underlying titanium substrate to the indium sulfite electrolyte solution 104, and the electrolyte solution 104 pH increases over time. The increase in pH destabilizes the electrolyte solution 104. As the pH of the electrolyte solution 104 increases, an associated increase occurs in indium concentration, due to chemical and galvanic dissolution of indium ions from the solid indium shot soluble anode. These indium ions exceed the complexing capacity of the electrolyte solution. The excess uncomplexed ions then precipitate as In(OH)3, which forms a sludge within the electrolyte solution. Precipitation of In(OH)3 leads to instability of the electrolyte solution and variations in the deposit morphology. During the electroplating process, the cathode 108 or wafer is lowered into the reservoir and brought into contact with the electrolyte solution 104, and a direct electrical current (applied at a specific amperage or voltage) is applied using the power supply 112, which can be either a rectifier or a battery.
FIG. 2 (prior art) is a drawing of an anode 200, which is an inert metal, such as titanium or platinized titanium, approximately circular with a central opening 206. Multiple smaller openings 208 are disposed within the anode 200 to provide a path for fluid flow. The anode 200 may include one or more attachment points 210 to allow connection of the anode 200 to an external power source in the apparatus 100.
If a platinum metal coating 204 is consumed during the electroplating process, then the platinized titanium anode 200 may require periodic replacement. Consumption of the coating 204 exposes an underlying titanium substrate 202 to the electroplating solution 104. The electrolyte solution 104 includes a solution of metal ions to be electroplated. The metal ions are introduced through dissolution of the soluble anodes or chemical addition of metal salts.
In the electroplating process, the anode 200 is placed within the electrolyte solution 104 in the apparatus 100. The electrolyte solution 104 is agitated, stirred or circulated to provide an even distribution of metal ions from within the electrolyte solution 104 across surfaces and edges of the anode 200 and wafer to be electroplated.
In arrangement of FIG. 2, the anode 200 maintains its dimensional integrity, and wafers are electroplated with a uniform thickness of metal.
Small inherent cracks and pores within the platinum metal coating 204 further increase the area to which the electrolyte solution 104 can contact with the titanium substrate 202. The titanium forms a galvanic cell with the indium pellets in solution.